1. Field of the Invention
This invention relates to improvements in sparking plugs and more particularly to a sparking plug having an improved contamination-proof capability.
2. Description of the Related Art
During an initial term before new cars are offered for sale to users, the engine of the new cars is usually trained repeatedly through conditioning running or short-time running in the condition of low-speed and low-load. The sparking plug has a ceramic insulator which has a leg exposed to the interior of the engine and during the training, the leg of the sparking plug mounted to the engine of a new car is contaminated with such a substance as carbon so that the sparking plug tends to undergo a failure to ignite. In order to guard the sparking plug against this type of initial contaminative damage, it has hitherto been practiced that a heat-proof insulating material is provided on the surface of the leg of the ceramic insulator exposed to the interior of the engine with the view of preventing leakage of current from the surface of the ceramic insulator leg to a metal body of the sparking plug.
In conventionally known techniques, the heat-proof insulating material is formed by applying, for example, silicone oil on the inner surface of the metal body and on the surface of the ceramic insulator leg (Japanese Patent Publication No. 51-38855) or by applying on the surface of the ceramic insulator leg a coating layer made of, for example, a mixture of silicone oil and paraffin or a mixed solution of fluorine system oil and a solution containing paraffin (Japanese Patent Publication Nos. 61-48225 and 62-46958).
Disadvantageously, however, the silicone oil has low viscosity and high fluidity and therefore the silicone oil or the coating layer formed of the mixture of silicone oil and paraffin, applied on the surface of the ceramic insulator leg by an amount necessary to fulfil the insulating effect, droops by its own weight and partly stays to bridge spark areas defining the spark gap of the sparking plug. Under these circumstances, the present inventors have found that when voltage is applied to the sparking plug having the spark areas bridged by the staying silicone oil in order to cause the spark areas to spark, the bridging silicone oil is carbonized by spark discharge energy and becomes electrically conductive to override the spark, ultimately giving rise to a failure to ignite the sparking plug.